The present invention relates to porous structures or tiles and methods of their preparation for use containing molten electrolyte in electrochemical cells. It is particularly adapted for use in fuel cells but may also have application within molten-salt secondary electrochemical cells. The tiles are of electrically insulating ceramics in the form of flat porous plates of sufficient porosity and pore size to retain molten electrolyte between the cell electrodes.
A fuel cell of the type contemplated can include a porous metal structure of such as a nickel alloy as the anode and a similar structure as the cathode in which a metal oxide such as NiO is formed during cell operation. Suitable metallic current collectors are included for support and electrical transmission from the electrodes. Bipolar cells can be stacked together with electrically conductive separators shaped to form gas manifolds for the fuel and oxidant gases. In these bipolar cells, the metal separators can form a wet seal with the electrically insulative electrolyte tile at the outer marginal portions of the tile.
A fuel cell employing molten alkali metal carbonate as electrolyte will typically operate at high temperatures of about 900-1000 K. to convert chemical energy to d.c. electricity. Fuels such as H.sub.2 and CO and oxidant gases such as O.sub.2 and CO.sub.2 react together during this conversion.
In cells of this type, the electrolyte tile is an important component in determining the cell life. A suitable tile must have good strength to withstand thermal cycles and pressure differences between anode and cathode compartments. The tile also must be dimensionally, structurally and chemically stable with the temperature changes which occur between operating and nonoperating intervals. It is important that the tile provide a good wet seal at the tile separator interfaces and that it should be crack-free to minimize cross leakage of fuel and oxidant gases between the electrode compartments. Furthermore, the electrolyte-filled pores within the tile should match the pore distribution of the electrodes in such a manner as to enhance wicking but without flooding of electrolyte into each electrode to maintain a large surface area for reaction.
Present molten carbonate fuel cells employ hot-pressed electrolyte tiles with little or no fixed bonding between adjacent ceramic particles within the tile structure. The liquid electrolyte is held by capillary forces within the compact of discrete particles.
Lithium aluminate is well suited from a chemical compatibility standpoint for use in molten carbonate fuel cells. However, porous tiles of lithium aluminate material previously have not been prepared with good structural, dimensional and chemical properties for use as electrically insulative electrolyte retainers. The hot-pressed electrolyte tiles are typically formed by blending particulate lithium aluminate and the cell electrolyte in the desired proportions and compacting the solid mixture at a temperature slightly below the electrolyte melting point. This procedure forms a compact with the electrolyte thoroughly dispersed among and around the ceramic particles. The particles are not fused or otherwise bonded together but merely are held by mechanical compaction. Electrolyte tiles of this type have incurred particle growth, precipitation, reaction with electrolyte and other structural changes due to cracking and plastic deformations to shorten cell life.
The completed tile structure formed by hot-pressing the inert support in mixture with the electrolyte is a solid, generally nonporous member that cannot be conveniently characterized by microstructural techniques such as metallography, mercury porosimetry and internal surface area measurements. Measurements of this type are of considerable assistance in selecting and matching electrolyte tiles to be used with the desired electrodes. In addition, these compacted tiles do not exhibit the desired strength for providing adequate long-life wet seals between the cell separators and for preventing cross leaking of gases between the electrode compartments.